lib: os: mpsc_pbuf: Do not use sem when CONFIG_MULTITHREADING=n

When multithreading is off packet buffer should not use feature which
allows waiting for available buffer.

mpsc_pbuf is used by logging which can work in deferred mode in
no multithreading build.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>